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'''[[Index fund]]''' is an [[investment fund]] that seeks to replicate the performance of a specified [[stock market index]], [[bond market index]], or other financial benchmark by investing in the securities that comprise the underlying index. Index funds are generally managed using a [[passive management|passive investment]] strategy, aiming to match rather than outperform the benchmark. They are typically characterized by broad diversification, relatively low operating costs, and low portfolio turnover. Index funds may be structured as [[mutual fund]]s or [[exchange-traded fund]]s (ETFs) and are widely used by retail and institutional investors for long-term investing, asset allocation, and portfolio diversification.

==History==

The concept of index investing emerged during the 1960s and 1970s through developments in [[modern portfolio theory]] and the [[efficient-market hypothesis]], which suggested that consistently outperforming the overall market through active stock selection was difficult over long periods.

One of the earliest institutional index funds was created in 1971 by [[Wells Fargo]] for pension fund clients. In 1975, [[John C. Bogle]], founder of [[The Vanguard Group]], introduced the [[Vanguard 500 Index Fund]], the first index mutual fund made widely available to individual investors. Initially receiving limited investor interest, the fund subsequently became one of the largest mutual funds in the world and contributed significantly to the growth of passive investing.

During the 1990s and 2000s, the introduction and expansion of [[exchange-traded fund]]s further accelerated the adoption of index investing. Today, index funds represent a substantial share of global investment assets.

==Investment strategy==

Index funds employ a [[passive management|passive investment]] strategy designed to replicate the performance of a benchmark index.

The strategy generally involves:

* purchasing the securities included in the benchmark index;
* maintaining portfolio weights similar to those of the underlying index;
* periodically rebalancing holdings following changes in index composition;
* minimizing trading activity to reduce costs.

Unlike actively managed funds, index funds do not attempt to select individual securities expected to outperform the market.

==Types==

Index funds may track a wide variety of financial benchmarks.

Common types include:

* broad market index funds;
* large-cap index funds;
* mid-cap index funds;
* small-cap index funds;
* sector index funds;
* international index funds;
* global index funds;
* bond index funds;
* commodity index funds;
* environmental, social and governance (ESG) index funds.

Some index funds follow domestic benchmarks, while others provide exposure to international or global financial markets.

==Replication methods==

Index funds generally employ one of several replication techniques.

===Full replication===

A full replication strategy involves purchasing all securities in the benchmark index in approximately the same proportions as the index itself.

===Sampling===

Where an index contains a large number of securities, the fund may purchase a representative sample designed to approximate the index's overall performance while reducing transaction costs.

===Synthetic replication===

Some index funds replicate benchmark performance through the use of financial derivatives, such as [[swap (finance)|equity swaps]], rather than directly holding all underlying securities.

==Advantages==

Index funds have several characteristics that have contributed to their widespread adoption.

These include:

* broad diversification;
* relatively low management fees;
* lower portfolio turnover;
* transparency of investment strategy;
* reduced transaction costs;
* tax efficiency in some jurisdictions;
* performance closely aligned with benchmark indices.

Because index funds generally trade less frequently than actively managed funds, they often incur lower operating expenses.

==Limitations==

Despite their popularity, index funds have several limitations.

Common criticisms include:

* inability to outperform the benchmark before fees;
* exposure to market declines;
* concentration in the largest constituent companies under capitalization-weighted indices;
* limited flexibility during periods of market stress;
* dependence on the methodology of the underlying index.

An index fund will generally decline in value if the benchmark index experiences a broad market downturn.

==Tracking error==

'''Tracking error''' refers to the difference between the performance of an index fund and that of its benchmark index.

Sources of tracking error may include:

* management expenses;
* transaction costs;
* cash holdings;
* dividend timing;
* sampling techniques;
* tax considerations;
* securities lending activities.

Fund managers seek to minimize tracking error through efficient portfolio management and periodic rebalancing.

==Applications==

Index funds are widely used for:

* long-term investing;
* retirement savings;
* institutional portfolio management;
* asset allocation;
* passive investment;
* diversification;
* wealth management;
* financial planning.

They are commonly used by pension funds, insurance companies, sovereign wealth funds, endowments, and retail investors.

==Relationship with exchange-traded funds==

Many [[exchange-traded fund]]s follow index-based investment strategies. However, not all ETFs are index funds, as some are actively managed or pursue specialized investment strategies.

Similarly, many index funds are structured as traditional mutual funds rather than ETFs. The distinction therefore relates to the legal structure and method of trading rather than the underlying investment strategy.

==Impact==

Index funds have significantly influenced global financial markets by promoting passive investing and reducing investment costs for both institutional and retail investors. Their growth has contributed to increased demand for diversified investment products linked to benchmark indices such as the [[S&P 500]], [[MSCI World]], [[FTSE 100 Index]], [[NIFTY 50]], and [[Nikkei 225]]. Index funds have also encouraged greater transparency in portfolio construction and have become central components of retirement plans, pension funds, and long-term investment strategies throughout the world.

==See also==

* [[Exchange-traded fund]]
* [[Mutual fund]]
* [[Investment fund]]
* [[Passive management]]
* [[Active management]]
* [[Stock market index]]
* [[S&P 500]]
* [[MSCI World]]
* [[NIFTY 50]]
* [[FTSE 100 Index]]
* [[Nikkei 225]]
* [[Asset allocation]]
* [[Portfolio management]]
* [[Modern portfolio theory]]
* [[Efficient-market hypothesis]]